1. Field of the Invention
The present invention pertains to manifold apparatus for sensing airflow, and, more particularly, to the sensing of airflow in a duct or controlled space utilizing a double manifold system.
2. Description of the Prior Art
The balancing of air duct systems used for heating and cooling purposes requires the measurement of air flows from duct outlets and inlets. The measurements are necessary in order to appropriately balance the airflow throughout the system. This may be accomplished by having some type of airflow sensing system installed permanently in a duct, or at several locations in a duct, or it may be accomplished by utilizing a portable sensor system. Most often, a portable airflow sensor system includes a hood which is disposed about the airflow outlet and which captures all of the airflow through the outlet or inlet and which directs the airflow through or across appropriate sensing apparatus where the airflow is measured.
Two measurements are generally taken of the flowing air. One measurement is that of the total pressure and the second measurement is static air pressure. The two measurements are then utilized to indicate the differential pressure, which represents the velocity pressure of the airflow. That is, the static pressure is subtracted from the total pressure to determine the differential pressure, which is velocity pressure of the airflow at the controlled environment of the sensor system.
It will be understood that while the present specification discusses air measurements in terms of pressure, pressures are or may be correlated to airflows, and thus meter readings are typically in CFM (cubic feet per minute) or any other unit of volumetric flow. Various other means may also be used to derive outlet flows being measured.
Since air duct outlets typically include diffusers for directing the air outwardly in a controlled manner, and the hood attempts to confine all of the airflow, even though it is directed in a plurality of different directions, the airflow in the hood is not in a laminar form. That is, the flow of air may be different at different locations within the hood. Moreover, turbulence, etc., may also be present within the hood. Accordingly, it is desirable to average the air velocities to provide a composite total of the flow of air.
Prior art devices typically include only a manifold disposed at the inner periphery of the flow sensing section, assuming that the flow at this inner peripheral area provides an averaging indication. However, such is not the case.
Another problem inherent in the prior art apparatus is the lack of ability to sense air velocities below about fifty feet per minute. The sensing of relatively low air velocity is difficult due to the relatively low velocity pressure, which is the difference between the total pressure and the static pressure, and the ability of instruments to accurately take readings at such low velocities and to provide accurate results from the sensing of the two pressures.
The apparatus of the present invention allows air at low flow rates to be sensed by providing a multiplier by using an apertured plate in conjunction with a manifold. The apertured plate provides a known net area or multiplier factor, thus enabling relatively low flow rates to be accurately measured.